Formaldehyde (HCHO) is carcinogenic and teratogenic and thereby is a serious danger to human health. Formaldehyde may be emitted from industrial processes and products. It is also one of the major indoor pollutants. Along with the improvement of life quality and health consciousness of people, the demand for methods for removing formaldehyde, especially indoor formaldehyde, is increasing in recent years.
The methods used can be classified into adsorption and decomposition. The adsorption is carried out by using porous materials such as active carbon, molecular sieves, zeolites and the like to immobilize formaldehyde. After being used for some time, the porous materials are saturated and thereby cannot adsorb formaldehyde anymore, which leads to the failure of the adsorption methods. In addition, the adsorbed formaldehyde may be desorbed due to a change of conditions such as rising of the ambient temperature, which leads to secondary pollutions.
Decomposition methods may include plasma decomposition and catalytic oxidation. The catalytic oxidation converts formaldehyde into water and carbon dioxide via catalysts, which is a promising technique. The key of such techniques is to develop suitable catalysts. In recent years, transition-metal oxide catalysts and noble metal catalysts have been developed. Manganese dioxide is a relatively abundant and inexpensive material. Accordingly, manganese oxide based catalysts are developed to aerobic oxidize organic compounds.
Manganese oxide containing catalysts are also known being useful for decomposing other hazardous substances. E.g., JP2014140816A disclosed a decomposing agent for chlorine dioxide preferably containing MnO2, Mn2O3, Mn3O4 (as manganese oxide) and M<I>MnO4 and M<II>MnO4 (as permanganate) (with M<I> an alkali and M<II> an earth alkaline metal). Quantitatively, 50 to 95% by weight of manganese oxide and 0.01-3% by weight of permanganate are preferred.
However, traditional manganese oxides like MnO2 have not enough activity to do the catalysis process on indoor formaldehyde at room temperature, since indoor formaldehyde generally present at a relatively low level only.
Accordingly, there is a demand for a catalyst which is able to effectively catalyze formaldehyde oxidation and thereby can be used in an air purifier to effectively remove indoor formaldehyde present at low level.